The invention relates to a new, efficient type-II interband cascade light emitting device and a method of making same. More particularly, this invention relates to a new, novel method of fabricating interband cascade light emitting. Devices made in this manner have record-high differential external quantum efficiencies, high peak output power and high power conversion efficiencies.
Compact, reliable mid-infrared (IR) (wavelengthxcex greater than 2.5 xcexcm, frequency v less than 120 THz) light sources are required for many civilian and military such as chemical sensing, IR communications, infrared countermeasures (IRCM), IR illumination, laser surgery, industrial process control, and IR lidar for aircraft and automobiles. If efficient semiconductor diode lasers were available for the mid-IR wavelength range, they would offer considerable advantages in terms of cost, volume, weight, simplicity of system design, reliability, and overall performance over presently available IR sources. Requirements for such lasers include a relatively high output power and operation either at ambient temperature or at temperatures accessible with thermoelectric (TE) coolers.
Three narrow band-gap material systems, i.e. the IVxe2x80x94VI lead salts, the II-VI ternary alloys, and the Sb-based III-V compounds, have been used as conventional interband transition approaches to mid-IR lasers. However, the poor qualities of these narrow band-gap materials and some inherent mechanisms have limited their performance. Generally, the small photon energy Ep (hv less than 0.5 eV) in the mid-IR spectral range is the cause of the inherent difficulties in obtaining high powers and in efficiently using applied voltage in conventional interband laser structures.
In 1994, a Bell Labs-Lucent team a quantum cascade (QC) laser, based on intersubband transitions in artificial semiconductor quantum-well (QW) structures, that has paved a new way toward high-power mid-IR light sources. The design had two major advantages over earlier longer wavelength diode lasers. First, its structure could be designed, in principle, to emit at any wavelength longer than about 5 xcexcm. Second, because it consisted of a unipolar active region, active layers could be stacked in series allowing for devices with higher quantum efficiencies. This means that after achieving lasing, each additional electron injected into the device could produce more than one photon; ideally, one photon for each active layer in the structure.
Intersubband QC lasers have relatively high threshold current densities (typically less than 1 kA/cm2) due to very fast phonon relaxation inherent to intersubband transitions, resulting in significant heating and subsequent limitations on power conversion (wall-plug) efficiency. Thus, it is very challenging for intersubband lasers to achieve continuous-wave (cw) operation and high average power at room temperature (the cw power efficiencies for such QC lasers have been reported in the literature as less than 9%, even at cryogenic temperatures). For applications such as IRCM that require high power constrained with specific size, weight, and power consumption limitations, power efficiency becomes the crucial figure of merit of the laser.
Interband cascade (IC) lasers, which utilize optical transitions between the conduction and valence bands in a staircase of Sb-based type-II quantum well (QW) structures, reuse injected electrons by taking advantage of the broken band-gap alignment in Sb-based type-II QWs to form cascade stages, leading to a quantum efficiency greater than the conventional limit of unity, similar to the intersubband QC laser. Such IC laser designs can circumvent the fast phonon scattering loss in intersubband QC lasers and suppress Auger recombination through band-structure engineering. Mid-IR IC lasers based on InAs/GaInSb type-II QWs are promising for obtaining high output powers.
It is an object of the present invention to provide an interband cascade (IC) light-emitting device capable of delivering high output powers and relatively high operating temperatures for use in a wide range of applications, while operating efficiently, as discussed above.
It is a further object of the present invention to provide a reliable and reproducible method of manufacturing a laser (light emitting device) having the above characteristics.
In order to achieve the above objects of the present invention, a first embodiment of the invention is provided consisting of a method of manufacturing an IC emitting device comprising the steps of (a) growing a laser sample from Al(In)Sb, InAs, and Ga(In)Sb layers having a top, bottom and cavity using a solid source molecular beam epitaxy (MBE) system on a (001)-oriented p-type GaSb substrate; (b) processing the laser sample into mesa-stripes having widths of from 35-215 xcexcm and about 500 xcexcm center-to-center separations between mesas using etching to form an etched sample;(c) depositing about 250 xcexcm thick SiO2 layer on the etched sample; (d) removing the 250 xcexcm thick SiO2 layer on top of the mesas to allow deposition of metal contacts to top device layers; (e) depositing Au/Ni/AuGe metal contact layers having a thickness of about 300 xcexcm and a width of about 250 xcexcm on the top of the sample to form a contact or bonding pad; (f) lapping and polishing the bottom of the laser sample down to about 100 xcexcm; (g) depositing a Ti/Au metal contact layer having a thickness of about 350 xcexcm on the bottom of the sample; (h) cleaving one or more laser bars having an epilayer and both facets left uncoated to the cavity of the laser sample; and (i) mounting the laser bars, epilayer side up, onto a second substrate to form the laser device.
A second embodiment is further provided consisting of a method of manufacturing an interband cascade light emitting device comprising the steps of (a) growing a laser sample having a top, bottom and cavity in a solid source molecular beam epitaxy (UBE) system on a (001)-oriented p-type GaSb substrate;(b) processing the laser sample into mesa-stripes having widths of from 35-215 xcexcm and about 500 xcexcm center-to-center separations between mesas using etching to form an etched sample; (c) depositing about 250 xcexcm thick SiO2 layer on the etched sample; (d) removing the 250 xcexcm thick SiO2 layer on top of the mesas to allow deposition of metal contacts onto the top device layers; (e) depositing Au/Ni/AuGe metal contact layers having a thickness of about 300 xcexcm and a width of about 250 xcexcm on the top of the laser sample to form a contact or bonding pad; (f) lapping and polishing the bottom of the laser sample down to about 100 xcexcm; (g) depositing a Ti/Au metal contact layer having a thickness of about 350 xcexcm on the bottom of the sample;(h) cleaving one or more laser bars having an epilayer and both facets left uncoated to the cavity of the laser sample; and (i) mounting the laser bars, epilayer side up, onto a second substrate. The IC light emitting device manufactured according to the process of the second embodiment is comprised of 18 active regions separated by n-type injection regions and a plurality of coupled quantum wells of Al(In)Sb, InAs, and Ga(In)Sb layers, and has a differential external quantum efficiency of at least 500%, a peak power output of at least 4W/facet, a power conversion efficiency of at least 14% in continuous wave at 80K, a power conversion efficiency of at least 18% in pulsed wave operation at 80K, a continuous wave operation temperature of 142K or less, a thermal resistance of from about 24-29 K*cm2/kW and continuous wave output powers of at least 100 mW/facet at temperatures above 77K.
In a third embodiment of the present invention according to the first embodiment above, the etching step (b) is performed using wet chemical etching and/or dry-etching techniques.
In a fourth embodiment of the present invention according to the second embodiment above, the deposition step (c) of a 250 xcexcm thick layer of SiO2 on the etched sample is performed using plasma enhanced chemical vapor deposition.
In a fifth embodiment of the present invention according to the third embodiment above, the deposition step (d) of Au/Ni/AuGe is performed using photolithography and/or lift-off techniques.
In a sixth embodiment of the present invention according to the fourth embodiment above, the second substrate is comprised of a metal.
In a seventh embodiment of the present invention according to the fourth embodiment above, the second substrate is a metal sheet.
In an eighth embodiment of the present invention according to the seventh embodiment above, the metal sheet is a thin copper sheet.
In a ninth embodiment of the present invention, an interband cascade light emitting device manufactured according to the method of the eighth embodiment above is provided, wherein the continuous output power of the device is at least 100 mW/facet at temperatures above 77K.
In a tenth embodiment of the present invention, an interband cascade light emitting device with lasing wavelengths of from about 3.6-3.8 xcexcm is provided, having 18 active regions separated by n-type injection regions and a plurality of quantum wells made of Al(In)Sb, InAs, and Ga(In)Sb layers.
In an eleventh embodiment of the present invention, an interband cascade light emitting device is provided according to the ninth embodiment above, further having a differential external quantum efficiency of at least 500%.
In a twelfth embodiment of the present invention according to the eleventh embodiment above, an IC light emitting device is provided having a peak power output of at least 4W/facet.
In a thirteenth embodiment of the present invention according to the twelfth embodiment above, an IC light emitting device is provided having a power conversion efficiency of at least 14% in continuous wave at 80K, and a power conversion efficiency of at least 18% in pulsed wave operation at 80K.
In a fourteenth embodiment of the present invention according to the thirteenth embodiment above, an IC light emitting device is provided having a continuous wave operation temperature of 142K or less.
In a fifteenth embodiment of the present invention according to the fourteenth embodiment above, an IC light emitting device is provided having a thermal resistance of from about 24-29 K*cm2/kW.
In a sixteenth embodiment of the present invention according to the fifteenth embodiment above, an IC light emitting device is provided having continuous wave output powers of at least 100 mW/facet at temperatures above 77K.
In a seventeenth embodiment of the present invention, a method of manufacturing an interband cascade light-emitting device is provided comprising the steps of (a) growing a laser sample made from Al(In)Sb, InAs, and Ga(In)Sb layers having a top, bottom and cavity; (b) processing said laser sample into mesa-stripes having widths of from 35-215 xcexcm and about 500 xcexcm center-to-center separations between mesas using etching to form an etched sample; (c) deposition of an about 250 xcexcm thick SiO2 layer on said etched sample; (d) removal of the 250 xcexcm thick SiO2 layer on top of said mesas to allow deposition of metal contacts to top device layers; (e) deposition of Au/Ni/AuGe metal contact layers having a thickness of about 300 xcexcm and a width of about 250 xcexcm on the top of said sample to form a contact or bonding pad; (f) lapping and polishing of the bottom of said laser sample down to about 100 xcexcm; (g) deposition of a Ti/Au metal contact layer having a thickness of about 350 xcexcm on the bottom of said sample; (h) cleavage of one or more laser bars having an epilayer and both facets left uncoated to said cavity of said sample; and (i) mounting of said laser bars, epilayer side up, onto a second substrate to form said device.